1. Field of the Invention
The present invention relates to a developing device for forming a developer image on an image carrier and an image forming apparatus using the developing device.
2. Description of the Prior Art
In the case of image forming apparatus, for instance, an electro-photographic copying machine, the downsizing of the main body is expected in order to save a required space. In particular, in the case of full-color electro-photographic copying machines, as a plurality of developing devices are used, the downsizing is eagerly demanded.
In the case of developing devices comprising such image forming apparatus, two conveying augers are horizontally arranged in two rows in the housing. These conveying augers are rotated and a two-component developer comprising toner and carrier is stirred, circulated, conveyed and supplied to a magnet roller.
However, when the augers are arranged in a row horizontally, a space required for installing the apparatus becomes large and therefore, the downsizing of the apparatus was achieved by arranging two conveying augers at upper and lower positions.
For instance, the Japanese Patent Disclosure (Kokai) No. 5-333691 discloses a developing device, which has a developer conveying route comprising two conveying paths; the upper and lower conveying paths by arranging a conveying auger at the upper and lower locations of a partition plate, respectively to convey developer received from the upper conveying path while stirring it and supply developer to a magnet roller and recover the developer from the magnet roller into the lower conveying path.
Now, the tip of the partition plate dividing the upper and lower conveying paths in the developing device is extended between the magnet roller and the upper conveying auger and the top edge height is kept at the same height over the entire length in its longitudinal direction and formed horizontally.
The magnet roller is arranged at a position higher than the upper conveying auger and therefore, developer is supplied to the magnet roller by the conveying pole of the magnet provided to this roller itself.
In other words, while developer is being conveyed, a part of the developer is supplied to the magnet roller. The developer not used by the magnet roller is conveyed to the lower conveying path and recovered without being returned to the upper conveying path, which is the source of supply of developer.
So, in the upper conveying path, the developer accumulate height becomes gradually low toward the downstream of the conveying direction along the longitudinal direction. On the contrary, in the lower conveying path the developer accumulate height tends to gradually rise toward the downstream in the conveying direction along the longitudinal direction.
By such a difference of the developer accumulate height along the longitudinal direction in the upper and lower conveying paths, supply amount of developer to the magnet roller becomes uneven. In order to solve this defect it is desirable that the magnet roller is provided at a point higher than the upper conveying auger. However, if this condition is realized, the developing device will become long and large in the direction of height.
If the position of the magnet roller is lowered in order to make the developing device small in size, supply amount of developer to the magnet roller is determined by the position of a doctor blade, which controls the thickness of the developer layer and an accumulate height of supplied developer in the upper conveying path.
If the conveying amount of developer by the upper and lower conveying augers is made sufficiently large, the effect of the lowered position of magnet roller is reduced. On the other hand, however, if the conveying amount of developer by these augers is increased too large even little, stress to developer increases and furthermore, running torque increases and therefore, it is not possible to increase conveying amount of developer too large (less than 3 times according to the technique disclosed in the Japanese Patent Disclosure (Kokai) No. 5-333691).
Accordingly, in the range for practical use, the developer accumulate height in the upper and lower conveying paths is still inclined along the longitudinal direction. That is, when the magnet roller is lowered, amount of developer to be supplied to the magnet roller is subject to the effect of the accumulate height of developer along the longitudinal direction.
At the upper stream side of the conveying path, where the accumulate height is high, amount of developer is much and image density becomes high and at the downstream side, where the accumulate height is low, amount of developer is less and image density becomes low. As a result, there is a problem that a difference in density is produced between the left and right sides of an image.